master01.dns.zen.net.uk has no CNAME records.
Question and response
Question
dig master01.dns.zen.net.uk. NS
Answer
The response has no answer records.
Authority
dns.zen.net.uk. 1751 SOA unamuno.zen.co.uk. dnsmaster.zen.co.uk. 777 1801 120 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0